The present invention is generally directed to a diagnostic tool for use in data processing and information transmission systems which communicate by means of optical fiber cable links. More particularly, the present invention is directed to a portable tool which is capable of consistent and reliable measurement of light power levels on optical fiber cables.
Because of their high capacity for information transmission, optical fiber links have become an important connection medium in data processing and information transmission systems. It is expected that the utilization of optical fiber links will continue to grow with increasing demand for higher bandwidth, especially as real time video transmission bandwidth needs become more significant. The improvements that have been made in optical fiber cables in recent years have also extended the distance over which information may be transmitted on such cables. This is a decidedly desirable result. However, the range requirements and other aspects of information transmission require the utilization of relatively powerful lasers to achieve the desirable information capacity and distance objectives. As a result of this and other factors, laser light emitted from one end of a fiber optic cable can pose known safety hazards. In particular, it is well known that such laser radiation may damage the human retina. Accordingly, in accordance with various standards, relating to the use of optical fibers in communication systems, it has become common practice to automatically shut down the laser transmission when the light path is interrupted. Typically, in relevant data transmission systems, optical fiber cables are disposed in duplex or multiplex fashion so that as soon as a light signal is interrupted at the receiving end, a shut down signal is sent back via other cable or cables to the originating information source to immediately shut down laser transmission. In the case of both fibers being interrupted, the lack of a return signal will also shut down the laser. Thus, when one "pulls the plug" at one end of a duplex fiber optic link, the utilization of at least one standard requires that the laser transmission cease. This provides a safety element which prevents laser radiation from impinging on biological tissues which might otherwise be damaged.
However, this safety requirement imposes a constraint which renders it exceedingly difficult to monitor the light power and/or to perform other diagnostic operations. In short, under a draft ANSI standard, its either "all or nothing" at one end of the fiber optic link. This standard is described as the open fiber control and is specified in the ANSI fiber channel standard document which specifies that the power is to be automatically turned off at the transmitter when the optical link is opened. This safety feature therefore prevents exposure to optical laser power levels that could be harmful that is, generally greater than international Class 1 limits. However, in order to isolate link problems and to perform maintenance, field service personnel need to measure optical transmitter output and receiver input levels. This would normally have to be done by breaking the link to perform the desired measurements. This is undesirable in that one or more of the processors is effectively idle during this period of time. Accordingly, there is a need to provide a portable tool for enabling the light characteristics to be measured during full link operation, in the face of the indicated safety requirements. The present invention is a tool particularly directed to solving this problem.
Optical splitters exist which are capable of "siphoning off" various percentages of light energy as it is transmitted down a fiber optic cable path. However, the use of commercially available optical splitters, even when disposed in a housing with stress relief, did not solve the problem of producing a usable portable tool. In fact, experiments conducted by one or more of the present applicants instead indicated a great deal of variability and inconsistency in the measured optical power level. Accordingly, the utilization of a simple splitter as a mechanism for providing optical energy for diagnostic purposes is not in and of itself acceptable. In particular, the present applicants have discerned that even simple movement or handling of the cable with the splitter, even in a housing with stress relief, still resulted in inconsistent measurements. Thus, a simple splitter, even one disposed in a stress relieved housing is an ineffective solution to the problem of measuring fiber optic light signals without interrupting the data flow provided by the optical signal.
While one form of the solution to the problem might be to place the fiber optic cables in a jig which holds them securely and prevents them from bending, moving or stretching in any way, such an arrangement is impractical and undesirable for a working portable tool. However, the applicants have perceived that the source of the problem is the sensitivity of the fiber optic cable to higher order mode re-distribution and loss which occur when the tool is used and/or handled. Additionally, splitters often use a fused biconic taper splice to strip off those light propagation modes nearer to the cladding of the optical fiber. These modes are in fact more-susceptible to fiber bending, micro bends, inclusions, micro scratches and other higher order mode re-distribution and loss. As a result of this, the present applicants have discerned that the unexpectedly high variability in optical power level measurements was in fact directly attributable to this sensitivity to higher order mode loss. Accordingly, the applicants herein have also discerned that the unexpectedly large variability can be corrected through the utilization of light conditioning. However, previously there was no indication of the need for any form of light conditioning in fiber optic cable applications which employ splitters.